The present invention relates to a method for self-calibrating a frequency of a modulator circuit, and circuit using said method, particularly but not exclusively a method for self-calibrating a frequency of center band of a band pass sigma-delta type modulator circuit.
The sigma-delta modulators, with high performance characteristics, are used in a various number of applications, such as, by way of example, in audio communication systems.
Particularly, said applications require high range dynamic characteristic, for example, by means of a digital resolution of twelve bits, a precise center band frequency, having a value of ten Mhz, a small physical dimension of the modulator and a low current consumption.
Sigma-delta type band passing multi bit modulators are used to reach said performance and physical characteristics.
Referring to FIG. 1, a basic scheme of a sigmaxe2x80x94delta modulator is shown, having a loop circuit 1 composed of an adding node 2, a go path 3 and a return path 4, also called feedback path.
In FIG. 1 it is evident that the go path 3 is realized by the series of a first block 5 with a second block 6, wherein said first block 5 is a filtering block and said second block 6 is an analog to digital conversion (ADC) block. The feedback path 4 is, instead, a digital to analog conversion (DAC) block.
The operation of the circuit scheme of FIG. 1 is well known to a skilled person and therefore will not be shown.
In the modulator 1, the filtering block 5, in the easiest implementation, may be made, by way of example, according to an integrator filter scheme of the first order, as shown in FIG. 2, which is well known to an individual skilled in the art.
As shown in FIG. 2, the filtering block 5 is composed by a block 7, having in the feedback path an amplifier 8.
Particularly the block 7 is a discrete filter having as transfer function H(z) the following relationship:
H(z)=a1*zxe2x88x921/(1xe2x88x92b1*zxe2x88x921)xe2x80x83xe2x80x83(1)
while the amplifier 8 has an opportune amplifying value suitable for the embodiment of the sigma-delta modulator.
Referring now to FIG. 3, an output spectrum from a modulator having, as a filtering block, a filter of the third order is shown, including an abscissa axis indicating the frequency evaluated in MHz, and an ordinate axis indicating the output noise spectrum evaluated in dB, a first graph 9, a second graph 10 and an observation band 12.
The first graph 9 is the ideal output spectrum of a third order modulator in the case of a pulsed input, in which the modulator has ideal characteristics.
In fact, inside of such a band 12, for example the band of FM signals, it is possible to note the presence of three minimum points 11, which represent the three notch frequencies introduced by each of the three discrete filters included in the go path of the loop of the modulator.
The second graph 10 is, instead, the actual output spectrum from the same third order modulator in the case of a pulsed input when the phase errors and gain errors introduced by the devices or by the process spreads introduce a sort of shifting of the ideal output spectrum 9 to the actual output spectrum 10.
It is just this shifting that produces the biggest problems of centering of the frequency.
As can be deduced from FIG. 3, when the integration operation is performed, an extra energy 13 is integrated, that is present over a selected threshold level 14.
Level 14 is the thermal noise level, always present in the modulator, and the extra energy 13 is therefore that part of energy that is integrated unnecessarily, degrading the dynamic range.
A structure as heretofore described with reference to FIG. 2, has problems caused by the non ideality of the integrator filter 7, which introduce phase errors and gain errors, depicted respectively by the coefficient b1 and by the coefficient a1 in the previous relationship (1), and gain and finite gain band product problems of the amplifier 8.
Various solutions have been proposed to solve such problems, among which are techniques based on the master-slave concept, wherein there is a duplication of the circuitry so as to calibrate the first circuit in function of the errors of the second circuit, and compensating circuit techniques of the finite gain effects.
However no technique has been able to prevent the previous listed factors from causing a shifting of the center frequency of the modulator 1 or an integration of undesired energy. Therefore, the problem remains of centering the center band frequency in the most precise possible way, without a degrading of the dynamic range.
According to an embodiment of the present invention, a method is provided, for calibrating a frequency of a sigma-delta modulator, said sigma-delta modulator having a go path and a feedback path, said go path including, in series, a resonator circuit and of an analog to digital conversion ADC block, said feedback path including a digital to analog conversion DAC block, said method comprising: a) applying an input pulse to said resonator circuit while the feedback path of said sigma-delta modulator is opened; b) measuring the oscillating frequency of the output signal from said resonator circuit in response to said pulse; c) comparing said oscillating frequency of said resonator circuit with a frequency known a priori; d) modifying said oscillating frequency of said resonator circuit proportionately, as a function of said comparing step (c).
According to another embodiment of the invention, a circuit for calibrating a frequency is provided, comprising a go path, including, in series, a resonator circuit and an analog to digital conversion ADC block, said resonator circuit comprising an integrator filter having, on its own feedback path, a variable gain amplifier. The frequency calibrating circuit further includes a circuit feedback path having a digital to analog conversion DAC block. Said variable gain of said amplifier is configured to be modified in a proportional way as a function of a comparison between an output signal frequency present at an output of said resonator circuit as a response to a pulsed signal present at an input thereof, and a frequency known a priori, while said circuit feedback path is opened.
Thanks to the present invention it is possible to make a measurement of a frequency within a shortened observation period, as compared to known methods and devices.